The method of making phenol by oxidation of cumene and catalytic cleavage of the resulting cumene hydroperoxide is well known and has replaced most of the phenol processes based on chlorination or sulfonation of benzene. More than five billion pounds per year of phenol capacity is now available in the world based on this technology.
A typical process consists of a cumene oxidation section, a CHP concentration operation, an acid-catalyzed cleavage reaction generally carried out in a fully back-mix reactor, and a product neutralization and recovery section.
Many improvements directed to increasing the yield and purity of the products have been disclosed in the patent literature. Also, many disclosed methods describe the use of heterogeneous catalysts for replacing sulfuric acid or other mineral acids which are now less desirable because of environmental regulations. Some improvements have also been made in the recovery of phenol, cumene and alpha-methylstyrene (AMS) from the waste products.
Methods using various acid catalysts are described in U.S. Pat. Nos. 2,718,172; 2,626,281; 3,187,052; 3,376,352; 4,016,213; 4,209,465; 4,267,380; 4,743,573; 4,870,217; 4,849,387; 4,490,565; 4,490,566; 4,898,987; and 4,898,995. Heterogeneous catalytic cleavage has to be carried out in either a fixed bed reactor or in a back-mix slurry reactor. As a result of the high exothermicity of the cleavage reaction and its relatively high rate of reaction, it is rather difficult to use a fixed bed reactor for this reaction. On the other hand, using a back-mix slurry reactor poses its own problems, namely, catalyst recovery and recycle, catalyst attrition, and mass transfer limitations at high conversions. In addition, catalyst deactivation can lead to unstable levels of CHP in the reactor which can lead to a runaway reaction.
Improvements to maximize the reaction yield have been primarily related to the reactions of the by-product AMS. It is formed in the cleavage reactor from the dehydration of dimethylbenzylalcohol (DMBA), which in turn is generally the largest by-product of the oxidation step. AMS is either recovered as a product or hydrogenated to cumene after it has been separated from phenol and acetone. Another major by-product is acetophenone which is usually not recovered. While acetophenone is inert under normal cleavage conditions, AMS can react with itself to form dimers or higher polymers or it can react with phenol to form cumyl phenol.
Where an increased yield of AMS is desired, it is maximized by maximizing DMBA conversion while minimizing the AMS side reactions. Methods for improving the AMS yield (minimizing side reactions) include adding acetone as a solvent to dilute the AMS or using alternative reactor configurations. Some of these methods are described in U.S. Pat. Nos. 2,663,735; 2,957,921; 2,757,209; 2,748,172; 3,376,352; 3,626,281; 3,187,052; 4,310,712, and 4,207,264. As these side reactions are consecutive and as the "desired" product AMS is the intermediate product, the reactor of choice for maximizing AMS yield is the plug flow reactor because of its low hold back value. This is incongruous with the ideal CHP cleavage reactor where it is desired to operate at a constant low CHP concentration and a high hold back value.
From these considerations was derived the concept of the two-stage cleavage reactor as described in U.S. Pat. Nos. 2,757,209 and 4,358,618. In these processes, CHP is incompletely converted in a first-stage back-mix reactor. Under these conditions, DMBA is only partially dehydrated to AMS so that the AMS concentration is kept low and its side reactions are minimized. Also, unreacted CHP condenses with DMBA to form dicumylperoxide (DiCup). The second-stage reactor consists of a tubular reactor operating essentially as an ideal plug flow reactor. In this reactor, residual CHP is cleaved to phenol and acetone, DiCup is decomposed to phenol, acetone and AMS, and residual DMBA is dehydrated to AMS. As a result of the kinetics of the plug flow reactor, higher yields of AMS can be obtained. However, the reactor must be designed for precisely the optimum residence time in order to maximize the AMS yield. Too short of a residence time will lead to unconverted DMBA and DiCup while too long of a residence time will result in conversion of additional AMS to heavies.
Numerous methods have been disclosed for removing by-products and purifying acetone and phenol produced from cumene oxidation. Some such methods are described in U.S. Pat. Nos. 2,748,172; 2,663,735; 2,790,549; 3,043,883; 3,140,318; 3,155,734; 3,187,052; 3,376,352; 3,965,187; 4,480,134; 4,626,600, and 4,722,769. These processes are primarily related to the removal of benzene from acetone and the separation of acetol, mesityl oxide, and other potential color bodies from phenol.
U.S. Pat. Nos. 2,715,145 and 4,960,958 report improvements in the recovery of phenol, cumene, and AMS from heavy ends. U.S. Pat. No. 3,441,618 discloses a method by which phenol distillation residue is selectively hydrogenated in order to produce better quality phenol, e.g. phenol with better stability toward discoloration when chlorinated. In this process, AMS is hydrogenated to cumene for recycle.
For each of these improvements, the basic concept of the CHP cleavage process remains the same. The goal is to maximize the yield of AMS regardless of whether the AMS is recovered as a separate product or hydrogenated to cumene and recycled. The maximum AMS yield lies within the range of 70-80% based on the DMBA in the feed to the cleavage step.
The present invention obviates many of the known problems of the cumene processes by providing higher phenol yields, less energy consumption, easier product purification, better product quality, and reduction of waste products including waste water.